High yield S-nitrosylation process

ABSTRACT

A method for producing a S-nitrosylated species is provided. The method comprises: (a) providing a deoxygenated, alkaline aqueous solution comprising a thiol and a nitrite-bearing species; (b) acidifying the solution by adding acid to the solution while concurrently mixing the solution (e.g., by vigorously stirring the solution) to produce the S-nitrosylated species; and (c) isolating the S-nitrosylated species. The nitrite-bearing species can be, for example, an inorganic nitrite, such as an alkali metal nitrite, or an organic nitrite, such as an alkyl nitrite (e.g., ethyl nitrite, amyl nitrite, isobutyl nitrite or t-butyl nitrite). The thiol is preferably a thiol-containing polysaccharide, a thiol-containing lipoprotein, a thiol-containing amino acid or a thiol-containing protein, and more preferably a thiol-containing polysaccharide such as thiolated cyclodextrin. In many preferred embodiments, the S-nitrosylated species is insoluble in the acidified solution, precipitating upon formation. The S-nitrosylated species can be isolated, for example, by a process in which the precipitate is removed from the solution (e.g., by centrifugation) and the aqueous solvent remaining in the precipitate is sublimated (e.g., by freezing the precipitate and subjecting it to a vacuum). The isolated S-nitrosylated product is preferably protected from heat, light, moisture and oxygen.

This Application is a continuation of U.S. patent application Ser. No.10/047,882, filed Jan. 14, 2002 now abandoned, entitled “High YieldS-Nitrosylation Process”, which is a continuation of U.S. patentapplication Ser. No. 09/645,171, filed Aug. 24, 2000 entitled “HighYield S-Nitrosylation Process, now U.S. Pat. No. 6,417,347, issued Jul.9, 2002.

FIELD OF THE INVENTION

The present invention relates to high-yield methods for producingS-nitrosylated compounds.

BACKGROUND OF THE INVENTION

S-nitrosylated compounds are compounds with one or more —S—NO groups. An—S—NO group is also referred to in the art as a sulfonyl nitrite, athionitrous acid ester, an S-nitrosothiol or a thionitrite.

Various S-nitrosylated species are known and include:

a) Certain S-nitrosylated polysaccharides, such as S-nitrosylatedstarch, cellulose, alginic acid, K-carrageenan, fucoidin, andcyclodextrins (e.g., α-cyclodextrin, β-cyclodextrin and γ-cyclodextrin).

b) Certain lipoproteins substituted at the S-moiety with a nitric oxidemoiety. Examples are chylomicron, a chylomicron remnant particle, a verylow-density lipoprotein, a low-density lipoprotein, anintermediate-density lipoprotein, a high-density lipoprotein and alipoprotein (a).

c) Certain S-nitrosylated polypeptides and proteins, such asneuropeptides, tissue-type plasminogen activator, streptokinase,urokinase, BSA, immunoglobulin (e.g., IgG, IgM, IgA, IgD, IgE),hemoglobin, myoglobin and cathepsin.

d) Certain S-nitroso amino acids and their derivatives (many of whichare ACE inhibitors) including S-nitroso-N-acetylcysteine,S-nitroso-glutathione, S-nitroso-cysteine,S-nitroso-gamma-methyl-L-homocysteine, S-nitroso-L-homocysteine,S-nitroso-gamma-thio-L-leucine, S-nitroso-delta-thio-L-leucine,S-nitroso-captopril, N-acetyl-S-nitroso-D-cysteinyl-L-proline,N-acetyl-S-nitroso-D, L-cysteinyl-L-proline,1-[5-guanidino-2-(S-nitroso)mercaptomethyl-pentanoyl]-L-proline,1-[5-amino-2-(S-nitroso) mercaptomethyl-pentanoyl]-4-hydroxy-L-proline,1-[5-guanidino-2-(S-nitroso)mercaptomethyl-pentanoyl]-4-hydroxy-L-proline,1-[2-aminomethyl-3(S-nitroso)-mercaptomethyl-pentanoyl]-L-proline, andS-nitroso-L-cysteinyl-L-proline.

e) Certain other S-nitrosylated compounds such asS-nitroso-penicillamine, S-nitroso-N-acetylpenicillamine, andY(CH₂)_(x)SNO, where x is 2-20 and Y can be —H, —SH, fluoro, C₁-C₆alkoxy, cyano, carboxamido, C₃-C₆ cycloalkyl, aralkoxy, C₂-C₆alkylsulfinyl, arylthio, C₁-C₆ alkylamino, C₂-C₁₅ dialkylamino, hydroxy,carbamoyl, C₁-C₆ N-alkylcarbamoyl, C₂-C₁₅ N,N-dialkylcarbamoyl, amino,hydroxyl, carboxyl, hydrogen, nitro or aryl; wherein aryl includesbenzyl, naphthyl, and anthracenyl groups.

Several of these S-nitrosothiol species have been noted for theirplatelet inhibition and/or thrombolytic characteristics, as well astheir ability to relax skeletal muscle and smooth muscle, includingvascular smooth muscle (vasodilation), airway smooth muscle,gastrointestinal smooth muscle, corpus cavernosum smooth muscle, bladdersmooth muscle, and uterine smooth muscle. For additional information,see, e.g., U.S. Pat. No. 5,770,645, U.S. Pat. No. 5,583,101, U.S. Pat.No. 5,863,890, U.S. Pat. No. 5,612,314, U.S. Pat. No. 5,648,393, U.S.Pat. No. 6,057,367 or U.S. Pat. No. 5,380,758, U.S. Pat. No. 5,593,876,U.S. Pat. No. 5,574,068 or U.S. Pat. No. 5,385,937, the disclosures ofwhich are hereby incorporated by reference in their entireties.

U.S. Pat. No. 5,770,645, the entire disclosure of which is incorporatedby reference immediately above, teaches that compounds with one or morefree nucleophilic groups, such as polysaccharides that have beenprovided with pendant thiol groups, can be reacted with a nitrosylatingagent under conditions suitable for nitrosylating the free thiol groups.Nitrosylating agents disclosed as suitable include acidic nitrite,nitrosyl chloride, compounds comprising an S-nitroso group(S-nitroso-N-acetyl-D, L-penicillamine (SNAP), S-nitrosoglutathione(SNOG), N-acetyl-S-nitrosopenicillaminyl-S-nitrosopenicillamine,S-nitrosocysteine, S-nitrosothioglycerol, S-nitrosodithiothreitol andS-nitrosomercaptoethanol), an organic nitrite (e.g. ethyl nitrite,isobutyl nitrite, and amyl nitrite), peroxynitrites, nitrosonium salts(e.g. nitrosyl hydrogen sulfate), oxadiazoles (e.g.4-phenyl-3-furoxancarbonitrile) and the like. For more information, seeU.S. Pat. No. 5,770,645.

SUMMARY OF THE INVENTION

According to an embodiment of the invention, a method for producing aS-nitrosylated species is provided. The method comprises: (a) providinga deoxygenated, alkaline aqueous solution comprising a thiol and anitrite-bearing species; (b) acidifying the solution by adding acid tothe solution while concurrently mixing the solution (e.g., by vigorouslystirring the solution) to produce the S-nitrosylated species; and (c)isolating the S-nitrosylated species.

The aqueous solution can be rendered alkaline, for example, by adding analkali metal hydroxide.

The solution can be deoxygenated, for example, by drawing a vacuum, bypurging with an inert gas (such as nitrogen), or by first drawing avacuum followed by purging with an inert gas.

The nitrite-bearing species can be, for example, an inorganic nitrite,such as an alkali metal nitrite, or an organic nitrite, such as an alkylnitrite (e.g., ethyl nitrite, amyl nitrite, isobutyl nitrite or t-butylnitrite).

The thiol is preferably a thiol-containing polysaccharide, athiol-containing lipoprotein, a thiol-containing amino acid or athiol-containing protein, and more preferably a thiol-containingpolysaccharide such as thiolated cyclodextrin.

Preferably, 1 to 2 equivalents of nitrite-bearing species are providedper thiol equivalent.

The acid can be, for example, an inorganic acid, such as hydrochloricacid, phosphoric acid or sulfuric acid, or an organic acid, such asacetic acid. Preferably, the acid is added to the solution in an amounteffective to provide at least 70% of theoretical yield within at least 1hour.

The S-nitrosylated species is preferably insoluble in the acidifiedsolution, precipitating upon formation. The S-nitrosylated species canbe isolated, for example, by a process in which the precipitate isremoved from the solution (e.g., by centrifugation) and the aqueoussolvent remaining in the precipitate is sublimated (e.g., by freezingthe precipitate and subjecting it to a vacuum).

The isolated S-nitrosylated product is preferably protected from heat,light, moisture and oxygen.

In one particularly preferred embodiment of the invention, a method forproducing an S-nitrosylated cyclodextrin species is provided. The methodcomprises: (a) providing a deoxygenated, alkaline aqueous solutioncomprising a thiolated cyclodextrin and a nitrite-bearing speciesselected from an alkali metal nitrite and an alkyl nitrite; (b)acidifying the solution by adding acid to the solution, whileconcurrently mixing the solution, to produce an S-nitrosylatedcyclodextrin species precipitate; and (c) removing the precipitate fromthe solution. 1 to 2 equivalents of nitrite-bearing species arepreferably provided per thiol equivalent. Moreover, the methodpreferably comprises sublimating any aqueous solvent remaining in theprecipitate after removal from the solution (e.g., by freezing theprecipitate and subjecting it to a vacuum). It is also preferable toprotect the sublimated S-nitrosylated cyclodextrin species precipitatefrom heat, light, moisture and oxygen.

One advantage of the present invention is that S-nitrosylated speciesare produced under conditions that minimize exposure to environmentalconditions that result in premature breakdown and NO release.

Another advantage of the present invention is that thiol species can benitrosylated in high yield.

Yet another advantage of the present invention is that a S-nitrosylatedspecies can be produced in the form of a fine powder, eliminating theneed for crushing or otherwise milling the product.

These and other embodiments and advantages of the present invention willbecome readily apparent to those of ordinary skill in the art uponreview of the Detailed Description and claims to follow.

DETAILED DESCRIPTION OF THE INVENTION

According to one aspect of the invention, S-nitrosylated species areproduced by first providing a deoxygenated, alkaline aqueous solutioncomprising a thiol and a nitrite-bearing species, and subsequentlyacidifying this solution by adding acid with concurrent vigorous mixing.

Essentially any nitrite-bearing species that is soluble in basicsolution and is capable of nitrosylating free thiol groups under acidicconditions can be used. Preferred nitrite-bearing species includeorganic nitrites and inorganic nitrites. Preferred organic nitrites arealkyl nitrites, more preferably C₁-C₈ linear or branched alkyl nitrites,such as ethyl nitrite, amyl nitrite, isobutyl nitrite and t-butylnitrite. Preferred inorganic nitrites are alkali metal nitrites such assodium nitrite, lithium nitrite, and potassium nitrite.

A wide range of base-soluble thiol species can be S-nitrosylated inaccordance with the present invention, so long as any undesirable sidereactions are kept to a minimum. Several S-nitrosylated products ofthiol species are discussed in the Background section above. Preferredthiol species include thiol-containing polysaccharides, thiol-containinglipoproteins, thiol-containing amino acids and thiol-containingproteins.

In some embodiments of the present invention, it is desirable to providea species with thiol groups for subsequent S-nitrosylation. For example,a thiolated species can be formed from a species having one or morependant nucleophilic groups, such as alcohols or amines. These pendantnucleophilic groups can be converted to pendant thiol groups by methodsknown in the art, such as those disclosed in Gaddell and Defaye, Angew.Chem. Int. Ed. Engl. 30:78 (1991) and Rojas et al., J. Am. Chem. Soc.117:336 (1995), the teachings of which are hereby incorporated into thisapplication by reference.

As a specific example, polysaccharides typically do not have thiolgroups, but do have pendant alcohol groups. For instance, particularlypreferred polysaccharides for the practice of the invention includecyclodextrins such as:

In accordance with the present invention, prior to S-nitrosylation, thepolysaccharide is first converted to a polythiolated polysaccharideusing, for example, the methods disclosed above. In these methods,primary alcohols are thiolated preferentially over secondary alcohols.In some preferred embodiments, a sufficient excess of thiolating reagentis used to form perthiolated polysaccharides. Polysaccharides are“perthiolated” when all of primary alcohols have been converted to thiolgroups. As seen from the above, alpha-cyclodextrin has six primaryalcohols, beta-cyclodextrin has seven primary alcohols andgamma-cyclodextrin has eight primary alcohols. Hence, perthiolatedalpha-cyclodextrin has six thiol groups, perthiolated beta-cyclodextrinhas seven thiol groups and perthiolated gamma-cyclodextrin has eightthiol groups.

Further details and a specific procedure in which beta-cyclodextrin canbe converted to per-(6-deoxy-6-thio)-beta-cyclodextrin are found in U.S.Pat. No. 5,770,645 (see, inter alia, Examples 1 and 2).

U.S. Pat. No. 5,770,645 also teaches that a polythiolated species can beprepared by reacting a polyhydroxylated species, and preferably theprimary alcohol groups of the polyhydroxylated species, with a reagentthat adds a moiety containing a free thiol or protected thiol to thealcohol. In one example the polysaccharide is reacted with a bisisocyanatoalkyldisulfide followed by reduction to functionalize thealcohol as shown in Structural Formula (I):

Conditions for carrying out this reaction are found in Cellulose and itsDerivatives, Fukamota, Yamada and Tonami, Eds. (John Wiley & Sons),Chapter 40, (1985) the teachings of which are incorporated herein byreference. One example of a polythiolated polysaccharide that can beobtained by this route is shown in Structural Formula (II):

Once obtained, the desired thiol and nitrite-bearing species arecombined in an alkaline aqueous solution.

Preferably, the solution is provided with 1 to 2 equivalents of nitritefor each thiol equivalent. The thiol bearing species is not necessarilyfully nitrosylated using this range of nitrite. For example, in the caseof perthiolated beta-cyclodextrin, which contains seven thiol groups permolecule, this range typically provides on the order of about oneS-nitroso group per cyclodextrin molecule.

In general, the solution should be sufficiently alkaline to preventpremature and significant nitrosylation of the thiol groups and toprevent precipitation of material under non-ideal conditions.Essentially any basic material can be used to render the solutionalkaline. Alkali metal hydroxides are preferred, most preferablylithium, potassium or sodium hydroxide.

While the solution should be sufficiently alkaline to preventsignificant nitrosylation of the thiol groups, additional base can beadded beyond this point, if necessary to assist in dissolving the thiolspecies. The appropriate pH to achieve each of these objectives willvary depending, for example, on the thiol species and nitrite-bearingspecies selected, but it can be readily be determined by those ofordinary skill in the art.

The order of addition of thiol, nitrite-bearing species and base isunimportant, so long as the solution is sufficiently alkaline at thepoint where the thiol becomes associated with the nitrite-bearingspecies in solution. For example, (1) the thiol can first be dissolvedin the basic solution, followed by the nitrite-bearing species, (2) thenitrite-bearing species can first be dissolved in the basic solution,followed by the thiol species, (3) the nitrite-bearing species and thethiol can be dissolved in the basic solution at the same time, and soforth.

As a specific example, a 0.1 NaOH solution (having a pH of about 13) canbe used as a starting solution, with thiol and the nitrite-bearingspecies added in turn.

Degradation of nitrosylated thiols (i.e., the release of NO andreversion to the thiol form) is accelerated upon exposure to oxygen,moisture, heat and/or light. Hence, in accordance with the presentinvention, it is desirable to reduce exposure to these conditions duringproduction and storage to the greatest extent possible.

In this connection and in accordance with the present invention, thesolution is deoxygenated prior to the formation of the S-nitrosylatedspecies (i.e., prior to acidification of the alkaline solutioncontaining the thiol species and the nitrite-bearing species). Asspecific examples, prior to acidification, (1) the basic solution canfirst be deoxygenated, followed by dissolution of the thiol species andnitrite-bearing species in the same (2) the thiol species andnitrite-bearing species can first be dissolved in the basic solution,followed by deoxygenation, (3) the thiol species can first be dissolvedin the basic solution, followed by deoxygenation and dissolution of thenitrite-bearing species, (4) the nitrite-bearing species can bedissolved in the basic solution, followed by deoxygenation anddissolution of the thiol species, and so forth.

A given solution can be deoxygenated by using any of several knownmethods. For example, the solution can be placed in a closed containerfollowed by (1) drawing a vacuum, (2) purging the overhead space with aninert gas, (3) bubbling the solution with an inert gas, (4) heating tonear boiling, and so forth. In one preferred technique, the solution isdeoxygenated by first drawing a vacuum. Then, the vacuum is eliminatedby purging the overhead space with an inert gas. Preferred inert gasesfor the above applications include nitrogen and argon.

Once a deoxygenated, basic aqueous solution of the thiol species and thenitrite-bearing species is obtained, the solution is subsequentlyacidified, preferably accompanied by vigorous mixing.

The acid is preferably added to the solution in an amount effective toprovide at least 70% of the theoretical yield within 24 hours, morepreferably within 1 hour, most preferably 10 minutes. The precise pHrequired to achieve this outcome will vary, depending on the thiolspecies and nitrite-bearing species selected, and it can be readilydetermined by those of ordinary skill in the art.

Essentially any acid can be used for this purpose so long as undesirableside reactions are kept to a minimum. Preferred acids for the practiceof the present invention include both inorganic acids, such ashydrochloric acid, phosphoric acid and sulfuric acid, and organic acids,such as acetic acid.

Vigorous mixing is carried out to minimize concentration gradients inthe solution while the nitrosylation reaction takes place. Mixing can berealized by known methods that include contacting the fluid with amoving member such as a stir bar or paddle, directing the fluid into astationary member such as a baffle, and so forth.

Various benefits can be garnered by mixing the solution, particularlywhere the reaction proceeds very quickly.

For instance, where multiple reaction sites are available, productuniformity is enhanced. As a specific example, perthiolatedbeta-cyclodextrin has seven available thiol sites for reaction. Based onthe reaction stoichiometry, however, only about one of these sites isnitrosylated on average. While not wishing to be bound by theory, it isbelieved that since this reaction proceeds rapidly, if concentrationgradients are allowed to persist in the solution after the acid is addedeven for a short period of time, the beta-cyclodextrin molecules in themore acidic environments will have more nitrosylated sites (due to morefavorable reaction kinetics) than those in less acidic environments.

As another example, where the resulting S-nitrosylated species isinsoluble in the acidified solution and precipitates from solution uponformation, a finer precipitate product is typically obtained byvigorously mixing during acid addition than would otherwise be obtainedin the absence of such mixing.

Each of the above procedures of the present invention is preferablyconducted at a temperature within the range of from about 0° C. to about50° C., more preferably at room temperature.

After the S-nitrosylated species is formed, it is preferably isolatedand dried to provide the product in powder form. In preferredembodiments of the invention, the S-nitrosylated species precipitatesfrom solution upon formation.

In particularly preferred embodiments of the present invention, thisprecipitate is first isolated in wet form from the aqueous solvent(e.g., by centrifugation), washed in deionized water and remainingsolvent in the wet sample removed by sublimation (e.g., lyophilizationor freeze drying). The sublimation process acts to protect the samplefrom degradation (e.g., because moisture and oxygen are removed), and itpromotes the formation of a very fine powder. As a specific example,where the precipitate is S-nitrosylated cyclodextrin, and where thesample is allowed to dry in air, substantial clumps form. In contrast,where the precipitate is dried by sublimation, a fine powder isproduced, and conditions are maintained which are less prone to degradethe final product.

Preferably, the wet precipitate is sublimated by first freezing it(e.g., by cooling the sample to −70° C. or by immersion in liquidnitrogen). The frozen precipitate is then placed in a container, and avacuum is drawn, whereupon the aqueous solvent is sublimated from thefrozen precipitate.

Once a dry product is formed, steps are preferably taken to avertpremature product degradation. For example, the product is preferablykept in a cool and dark place, such as a refrigerator or freezer. Foradditional protection, the environment surrounding the sample ispreferably an inert, water-free environment, and is more preferably avacuum environment or a desiccated, inert gas environment.

EXAMPLE

4 grams of beta-cyclodextrin thiol are initially provided.(Beta-cyclodextrin thiol can be prepared for example, using theprocedures of Examples 1 and 2 of U.S. Pat. No. 5,770,645, the entiredisclosure of which is incorporated by reference.) Then, a 300 mlportion of 0.1 N NaOH (which has a pH of approximately 13.0) is added tothe beta-cyclodextrin thiol. After mixing to dissolve the cyclodextrinthiol, the solution is filtered through a 0.45 micron filter. 1.6 gramsof sodium nitrite (NaNO₂) are then added. After mixing to dissolve thesodium nitrite, the solution is divided into 25 ml aliquots. For eachaliquot, the following procedure is performed:

1. The solution is degassed by applying a vacuum, after which theremoved gas is either replaced with nitrogen or the solution is keptunder vacuum.

2. Under nitrogen or vacuum, 300 ml of 0.1 N HCl are quickly injectedinto the solution accompanied by vigorous stirring.

3. The mixture is reacted for 5 minutes, over which time a pinkprecipitate is formed.

4. The mixture is then centrifuged at 4000 rpm (approx. 800×g) for 10minutes.

5. The supernatant is removed.

6. The precipitate is washed by suspending it in degassed de-ionizedwater, followed by centrifugation and recovery of the precipitate.

7. After repeating the prior washing step three times, the recoveredprecipitate is immediately frozen in liquid nitrogen for 5 minutes.

8. The frozen material is then lyophilized (sublimated under vacuum)until dry (typically over a period of several days).

9. Once dry, the material is stored under nitrogen at −20° C.

Although various embodiments are specifically illustrated and describedherein, it will be appreciated that modifications and variations of thepresent invention are covered by the above teachings and are within thepurview of the appended claims without departing from the spirit andintended scope of the invention.

What is claimed is:
 1. A method for producing an S-nitrosylated speciescomprising: providing a deoxygenated, alkaline aqueous solutioncomprising a thiol and a nitrite-bearing species; combining saidsolution with an acid with concurrent mixing to produce saidS-nitrosylated species; and isolating and drying said S-nitrosylatedspecies to form a powder.
 2. The method of claim 1, wherein said aqueoussolution is rendered alkaline by combining said thiol with a basicmaterial.
 3. The method of claim 2, wherein said basic material is analkali metal hydroxide.
 4. The method of claim 1, wherein said solutionis deoxygenated by drawing a vacuum.
 5. The method of claim 1, whereinsaid solution is deoxygenated by purging with an inert gas.
 6. Themethod of claim 5, wherein said inert gas comprises nitrogen or argon.7. The method of claim 1, wherein 1 to 2 equivalents of nitrite-bearingspecies are provided per thiol equivalent.
 8. The method of claim 1,wherein said nitrite-bearing species is an inorganic nitrite.
 9. Themethod of claim 8, wherein said inorganic nitrite is an alkali metalnitrite.
 10. The method of claim 1, wherein said nitrite-bearing speciesis an organic nitrite.
 11. The method of claim 10, wherein said organicnitrite is an alkyl nitrite.
 12. The method of claim 1, wherein saidacid is an inorganic acid.
 13. The method of claim 1, wherein said acidis an organic acid.
 14. The method of claim 1, wherein said thiol isselected from a thiol-containing polysaccharide, a thiol-containinglipoprotein, a thiol-containing amino acid and a thiol-containingprotein.
 15. The method of claim 14, wherein said thiol-containingpolysaccharide is thiolated cyclodextrin.
 16. The method of claim 1,wherein amid S-nitrosylated species is insoluble in said acidifiedsolution and forms a precipitate.
 17. The method of claim 16, whereinthe S-nitrosylated species is isolated and dried by a process comprisingcentrifuging and drying said S-nitrosylated species to form said powder.18. The method of claim 16, wherein the S-nitrosylated species isisolated and dried by a process comprising: removing said precipitatefrom solution and sublimating aqueous solvent remaining in saidprecipitate.
 19. The method of claim 18, wherein said sublimating stepcomprises freezing said precipitate and subjecting said precipitate to avacuum.
 20. The method of claim 1, wherein said S-nitrosylated speciesis isolated and dried by a process comprising a sublimating step.